The invention relates generally to electromagnetic shielding, and more specifically to an electromagnetic compatibility shielding system for electronic devices.
Computerized systems and many other types of electronic devices contain circuits that carry signals changing at high speed. As the circuits operate and the state of the circuits change, the circuit may emit electromagnetic radiation. This radiated electromagnetic radiation may potentially interfere with other circuits or radio communication, and so standardized limits for electromagnetic radiation are enforced in the United States and many other countries.
Current generations of computer systems can include digital circuits operating at a clock rate of over one gigaHertz and dissipating over 150 watts of power, resulting in digital signals with significant energy. These digital signals have transition times significantly faster than the clock rate, and so will include substantial energy of much higher frequency than the system clock frequency. This electromagnetic radiation is typically contained by shielding the computer or other electronic device, such as by enclosing the components in a suitable case with electromagnetic shielding capability.
Such electronic device cases or enclosures usually comprise more than one part, such as an enclosure with a removable panel or lid to allow configuration or servicing of the electronic device. Ensuring proper shielding requires that any gaps in the enclosure be well sealed, with progressively smaller gaps being permissible as higher frequencies must be shielded. For example, the wavelength of an electromagnetic signal at 1 gHz is under one-third of a meter, and any 100 gHz energy present in the transitions of a 1 gHz digital signal is only a third of a centimeter in wavelength. To effectively shield signals with these short wavelengths, gaps with dimensions approaching the wavelength of the signals to be shielded must be filled or broken up.
While it is likely inevitable that some gaps will remain in enclosures used for shielding due to factors such as cooling and ventilating, external interfaces mounted in the enclosure, and imperfect sealing of the enclosure, it is desirable to reduce gaps in the enclosure to less than one-twentieth the wavelength of the highest frequency to be shielded.
Solutions to the enclosure sealing problem have traditionally included series of leaf springs that are forced into contact between an enclosure and a removable panel, use of semiflexible and conductive gaskets that are attached to either the removable panel or the enclosure, or use of punched indentations in either the enclosure or removable panel that are forced into contact as the panel seals. Each of these solutions has various advantages and disadvantages with respect to cost and effectiveness, but none of the solutions provide a solid electrical and mechanical contact with low impedance at a low material and labor costs.
What is needed is an improved electromagnetic shielding system providing a relatively inexpensive seal between elements of an enclosure that provides solid contact and a low impedance connection.